The present invention relates to control methods for operating a power converter circuit for use in automobiles and other vehicles.
Due to consumer demand for comfort and convenience and complex control systems required to meet governmental regulation, electrical demands on modern automobiles have increased substantially over the years. Many consumers desire automobiles from which they can operate external accessories, such as laptop computers, and which provide a high power output to operate several accessories at once. In order to accommodate this desire, the automobile must contain an efficient power converter circuit, which converts a low input DC voltage from the automobile battery into a single phase 120V AC 60 Hertz sinusoidal output voltage with sufficient power to operate the external accessories.
Conventional converters consist of two stages: a DC/DC converter and a DC/AC inverter. Specifically, full bridge DC/DC converters with conventional pulse width modulation (PWM) current and voltage mode control, full bridge DC/DC converters with zero voltage transition phase shifted control, resonant mode DC/DC converters, current source DC/DC converters, and devices that convert low input DC voltage to a single phase 120V AC voltage are known.
Typically, full bridge DC/DC converter circuits are power switching circuits that have four transistors connected in a bridge configuration to drive a transformer primary. These DC/DC converters operate by high frequency switching action employing inductive and capacitive filter elements and are often controlled by pulse width modulation (PWM). PWM is a switching power conversion technique where the width of a duty cycle is modulated to control power transfer. A pulse width modulator is generally an integrated discrete circuit used in switching-type power supplies, to control the conduction time of pulses produced by a clock.
These DC/DC converters and DC/AC inverters operate certain ways, such as the simultaneous switching ON and OFF of two pairs of MOSFETs in a full bridge DC/DC converter, where one pair of MOSFETs alternates with the other pair, or fluctuating the duty cycle in the DC/DC converter to maintain a fairly constant DC output at the first stage. The secure method of operation has certain disadvantages. These simultaneous switches and fluctuating duty cycle result in high switching losses, high circulating currents, and undesirable voltage notches and ringing, resulting in reduced efficiency and increased noise. It is the objective of the present invention to provide a method of operating a power converter circuit to overcome these disadvantages.
The power converter circuit of the present invention includes two stages. The first stage is an isolated full bridge DC/DC converter circuit that converts the low input DC voltage to an isolated high voltage DC. The second stage is a full bridge DC/AC converter circuit that converts the high voltage DC to a single phase 120V AC 60 Hertz sinusoidal output voltage. Each stage has its own control circuit (one for the DC/DC converter and one for the DC/AC inverter).
The two control circuits used in the present invention incorporate four methods of operation. First, the DC/DC control circuit operates the DC/DC converter by xe2x80x9csoft switching.xe2x80x9d Instead of switching each pair of MOSFETs OFF simultaneously, the method of the present invention switches each pair of MOSFETs OFF at different times. The DC/DC control circuit also provides full PWM control and xe2x80x9cSynchronous Switchingxe2x80x9d of the full bridge DC/DC converter of the first stage. This operation results in reduced circulating current and reduced voltage ringing, resulting in increased efficiency.
Second, the DC/DC control circuit of the present invention keeps the duty cycle relatively constant and lets the DC/DC converter output voltage fluctuate in proportion to fluctuation of the input voltage. By setting the duty cycle at a constant and relatively high level, the switches are ON more often than they are OFF. This operation results in less average circulating current and improved efficiency.
Third, the DC/DC control circuit of the present invention operates the converter in current source mode. Operation of the DC/DC converter in current source mode prevents the DC/AC inverter""s second harmonic circulating current from flowing back through the DC/DC converter stage. This operation reduces the main device losses and current stresses in the DC/DC converter circuit, reduces input filtering requirements, and reduces noise.
Fourth, the DC/AC inverter control circuit of the present invention provides inverter protection control for the second stage. The inverter protection circuit provides a real time current limit protection scheme that measures and evaluates the current in the DC/AC inverter circuit every switching cycle. Under excessive current conditions, the inverter protection circuit turns OFF the transistors in the DC/AC inverter. Although the circuitry for operating a current limit protection scheme is known, it has not been applied on a cycle by cycle basis to second stage DC/AC inverters in a power converter circuit.
Use of these control methods in this manner provides numerous advantages. The DC/DC converter control circuit of the present invention achieves low harmonic distortion of output voltage waveforms with excellent voltage regulation over a wide range of loads. In addition, it lowers switching losses, noise, and circulating current.